No. I.] THE LESSER PERITONEAL CAVITY. 175 



a corresponding increase of the lesser peritoneal cavity, which 

 now lies on the dorsal side of the embryonic stomach. Besides 

 the elongation of the mesentery, there is also a gradual shifting 

 of the stomach away from the mouth. After the liver and heart 

 are well formed they lie in a straight line, in a transverse sec- 

 tion, the liver being between the stomach and the heart. These 

 three organs now lie immediately on the ventral side of the first 

 dorsal vertebra. In the adult the stomach lies below the twelfth 

 dorsal. In the dog six millimetres long there is as yet no artery 

 to these organs, while in the human, seven millimetres, the artery 

 is well formed, and arises from the aorta, just opposite the fourth 

 dorsal. The omphalo-mesenteric artery, i.e. superior mesenteric, 

 arises one segment deeper. In two dogs ten millimetres long 

 the cceliac axis arises opposite the second and fourth dorsal 

 segments respectively, while in a dog, 13.5 millimetres long, it 

 lies as low as the tenth. In a cat twelve millimetres the cceliac 

 axis lies opposite the tenth dorsal, and the omphalo-mesenteric 

 artery opposite the eleventh dorsal. In His's embryo M the 

 omphalo-mesenteric arises opposite the first dorsal, 1 while in 

 embryo B the same vessel arises opposite the fifth dorsal. 2 In 

 embryo A the cceliac axis arises opposite the sixth dorsal, and 

 the omphalo-mesenteric opposite the ninth dorsal. 3 



These cases are sufficient to show that the stomach gradually 

 moves away from the mouth, carrying its artery with it. Addi- 

 tional evidence is obtained from the study of the development 

 of the splanchnic nerve. When the cceliac axis is opposite the 

 first and second dorsal vertebrae, there are twigs of sympathetic 

 nerves which surround it, coming from the first and second 

 dorsal nerves. As the artery descends, the nerves following, 

 third, fourth, etc., are, one after another, included, thus account- 

 ing for the high origin of the splanchnic. 4 



In a dog ten millimetres long the general shape of the lesser 

 peritoneal cavity corresponds with the outline of the stomach. 



1 Atlas, PL VII, M 4; PI. VI, Fig. 15. 



2 Replace Fig. 24, PI. Ill, Atlas, into Fig. I, PI. I. 



3 Replace Figs. 79 and 86, PI. V, Atlas, into Fig. 4, PI. I. 



4 Although the splanchnic is usually spoken of as arising from the eighth dorsal, it 

 may be traced up to the third, second, or even the first dorsal. — Beck, Phil. Trans., 

 1846. 



The change of the relation of the spinal column to the heart is shown in a figure 

 by Uskow, Arch, fiir tnikrosk. Anat., Bd. 22, S. 1 88. 



